As a fuel with low CO2 emissions, natural gas has experienced a major expansion worldwide. When distance and topography between gas fields and consumer markets do not allow pipeline transport, the gas can be reduced to, for example, 1/600 of its free volume by liquefaction. Liquefied Natural Gas (LNG) plants liquefy purified natural gas in cryogenic heat exchangers so that the purified liquid natural gas can be stored in tanks prior to being loaded on designed tankers for transportation between an LNG plant and consumer regions. At the consumer regions, the LNG product is unloaded in an LNG receiving terminal, pumped up to pipeline pressure, and re-gasified for feeding into the buyer's natural gas pipeline grid.
In response to these increased demands, the size of LNG plants has, therefore, grown considerably over the past few decades. This, in turn, has contributed to lowering LNG production costs, while at the same time increasing the competitivity of the LNG marketplace. For example, in the 1980s, it was common practice to produce 2-3 million tons per steam per year. Present units produce 4-5 million tons per year, and engineering companies are now (i.e., 2009) planning plants with unit capacities in the order of 7-8 million tons per year.
In LNG plants, it is common practice to use, for example, gas turbines to drive the refrigerating compressors utilized in the cryogenic heat exchangers responsible for the liquefaction of natural gas. Thus, production output may be closely associated with reliable plant operation, in particular, gas turbine reliability. The reliable operation of gas turbine systems may be hampered by a myriad of different failure causes. As described in the following paragraphs, one such cause is the adverse effect of corrosion on the Inlet Guide Vanes (IGVs) and rotor blades of the gas turbine's turbo compressor.
LNG plants are typically located in marine coastal environments, where corrosive elements such as Chlorides and Sulphides are prevalent in the atmosphere. The Chlorides originate as a result of proximity to the sea, while the Sulphides are generated by the LNG plant's gas flares. The air filtration system of a gas turbine needs both correct design and maintenance, especially since it provides the key to the successful operation and reliability of the overall plant by purifying air that is inlet into the combustion section of the gas turbine system. Despite attempts to maintain reliable and effective air filtration, the presence of corrosive elements such as Chlorides and Sulphides in the various stages of the gas turbine system, such as, the axial compressor blades of the gas turbine (e.g., IGVs and R1 rotor blades) is unavoidable. These elements (i.e., Chlorides and Sulphides) may, for example, corrode the material construction of the gas turbine's IGVs and first stage (R1) rotor blades by causing corrosion pitting, which if undetected, eventually leads to the initiation and propagation of cracks within the blades. The consequences of such cracks are breakages in one or more of gas turbine's IGVs and R1 rotor blades, thus, causing an eventual gas turbine outage.
These outages are extremely costly. Usually, there is zero-redundancy associated with the production machinery/equipment of LNG plants. Within a given gas turbine train, the outage of one gas turbine can cause an overall train outage or, at least, a vast reduction of LNG production rate. As a consequence, LNG shipment may also be postponed, generating additional costs and/or profit losses that may be estimated in the range of $2-$7 Million Dollars/day depending on the specific plant size and production plan. For this reason all catastrophic failures, such as those generated by corrosion pitting, should be avoided since an outage of 7-10 days duration is the expected duration for restoring the system back to an operational status.
Thus, in order to avoid a gas turbine outage situation as a result of undetected failure conditions, it would be advantageous to provide in-situ corrosion detection within LNG gas turbine systems without the need for disassembling the gas turbine system (e.g., casing).